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Can drugs induce psychosis? Psychiatrists are divided, although psychotic patients who claim that their drinks have been “spiked” are not uncommon. It’s a way of understanding why you feel strange or have had hallucinations, especially in the UK, where an estimated half a million ecstasy tablets are taken every week. The chief suspect for causing psychosis amid classified drugs, however, is cannabis. The supposed link between cannabis and schizophrenia seems to be a major plank in the argument against decriminalising drugs, although in the rogues’ gallery of health risks cannabis hardly compares to nicotine, alcohol or even refined carbohydrates.

The problem with drugs is that they come in all sorts of unregulated, and thus different, forms. Cannabis in particular has lots of ingredients. Whether you call it weed, ganja, spliff, green or puff, the proportion of THC-3 (the stuff that alters your mind) or cannabidiol, the muscle relaxant and anxiety-relieving component, is literally pot luck.

More organised growers in jurisdictions where some consumption is allowed (such as in Amsterdam, or Colorado, where cannabis can be prescribed for medical or recreational use) can work to standardise their drugs, but the variability conundrum affects research into schizophrenia. What is causing which psychological effect? Despite a surge in UK cannabis use over the past 30 years, no increase in our incidence of schizophrenia has been reported. Cannabis-endemic countries such as Jamaica likewise report no increase. Studies in New Zealand, however, indicate a slight risk if you smoke cannabis heavily in your teenage years.

Many patients with chronic paranoid schizophrenia live rather limited lives. We can keep them out of hospital, almost always thanks to regular injections or other medications; their hallucinations can be put on the back burner and paranoia quelled. But sedating side-effects are common, because part of the illness (crudely described) is that the sufferer’s dopamine and/ or serotonin drive systems are distorted and overactive. Damp these down and you feel calmer but you also lack “get up and go”.

John, in his thirties, led a quiet life after two or three admissions under the Mental Health Act when younger. He went to the local mental-health day centre; he watched videos, played a bit of pool and saw his mum for Sunday lunch. When he relapsed for the first time in four years, we asked him why, and checked out his habits. John was quite open. He insisted that he hadn’t stopped his medication, and talked about his ganja use. He smoked it regularly, two or three joints a day, especially at night to help him sleep. It didn’t make him feel paranoid. If anything it made him feel “chilled” and able to enjoy things – a pleasant release from a rather sterile world.

A somewhat puritan trainee suggested that John’s relapse was “drug induced” and thought he should stop his habit. But why would he have relapsed? Maybe he had become more sensitive to cannabis. Maybe it was a stronger type. Some patients do leave hospital and relapse quickly in a haze of aromatic smoke. But for John the answer proved simple. A social worker checked his medications at home and found he’d been given the wrong dose: a prescription error, a harassed locum GP and thus the crash. John smiled, thanked us, got better and went back to his regular life.

Trevor Turner is a consultant psychiatrist working in east London. Some details have been changed to protect identity